A "no-travel" foot switch is one in which the manually actuated part carries out little or no physical movement when actuated. Traditionally, foot switches of this type were limited to switches capable only of on/off operation. In order to have an output signal which varied progressively in response to progressively increasing manual operation, it was necessary to have a manually actuated part which moved through a range of physical movement and was connected to a rheostat or the like.
A relatively new component known as a force sensing resistor, which is itself conventional for purposes of the present invention, has been used to provide a no-travel foot switch with a progressively variable output. More specifically, the force sensing resistor is a relatively flat element with two terminals, and progressively varies a resistance between the terminals in response to progressively increasing compressive forces in a direction perpendicular to the plane of the element.
A known foot switch using a force sensing resistor has the force sensing resistor disposed on an upwardly facing surface of a base member, and has a layer of firm rubber of uniform thickness provided over the upwardly facing surface and the force sensing resistor. To operate the foot switch, a foot is placed on the upper side of the rubber layer and pressed downwardly. The rubber layer does not move to any perceptible extent, thus providing a no-travel feel, but a progressively increasing force acting through the rubber layer onto the force sensing resistor causes the force sensing resistor to vary its resistance as a function of the pressure exerted. In this conventional arrangement, the resistance of the force sensing resistor is normally detected by connecting the force sensing resistor in series with another resistor between ground and a DC voltage, and then monitoring the voltage at the node between the resistors. While this approach has been generally adequate for its intended purposes, it has not been satisfactory in all respects.
More specifically, if the foot is precisely centered over the force sensing resistor when engaging the rubber layer, then a given amount of pressure will produce a given resistance value from the force sensing resistor, whereas if the foot is somewhat offset from the centered position on the rubber layer, some or all of the applied force will act through the rubber layer onto the surface of the base member rather than onto the force sensing resistor, as a result of which the same given pressure will produce a significantly different resistance from the force sensing resistor. Consequently, the foot switch can be somewhat difficult to use in a manner which is reliable and consistent.
A further consideration is that the above-mentioned sensing arrangement introduces a nonlinearity. In particular, the variation of the resistance of the force sensing resistor changes the overall resistance of the two series-connected resistors, and thus changes the amount of current flowing through both resistors, which in turn introduces into the output signal from the sensing circuit a nonlinearity with respect to the pressure applied, above and beyond any nonlinearity which may be inherent to the force sensing resistor itself. Further, the sensing resistor must typically be given a value large enough to ensure that the maximum current through the force sensing resistor does not exceed a maximum allowable current level, which in turn can increase the susceptibility of the sensing arrangement to noise.
Accordingly, one object of the present invention is to provide an improved foot switch of the type using a force sensing resistor, which minimizes or eliminates the effects of misalignment of a foot with respect to the foot switch, in order to provide a reliable and consistent output in response to a given amount of applied pressure.
A further object is to provide a sensing arrangement for a foot switch of the type using a force sensing resistor, which avoids introducing nonlinearities beyond those inherent to the force sensing resistor itself, which provides a high degree of noise immunity, and which ensures that the current through the force sensing resistor will never exceed a maximum allowable level.